Install Rails 3
 sudo gem install tzinfo builder i18n memcache-client rack rake rack-test rack-mount erubis mail text-format thor bundler
 sudo gem install rails --prerelease

Create a new Rails app
 rails -d mysql new_model
 cd new_model
 rails generate model Post title:string body:text published:boolean
 rails generate model Comment body:text post_id:integer
 rake db:create
 rake db:migrate
 rails server

New modules extracted from ActiveRecord
 ActiveModel
   Validation ie.
     include ActiveModel::Validations
     validates_presence_of :first_name, :last_name
     attr_accessor :first_name, :last_name

   Serialisering
     include ActiveModel::Serialization
     lav atttributes-metode, der returnerer array

   Callbacks
   Observing: ActiveRecord-style observers

   AttributeMethods: Makes it easy to add attributes that are set like table_name :foo
   Callbacks: ActiveRecord-style lifecycle callbacks.

   Dirty: Support for dirty tracking

   Naming: Default implementations of model.model_name, which are used by ActionPack (for instance, when you do render :partial => model

   Translation: The core translation support

New ActiveRecord query interface in Rails 3
 published = Post.where(:published => true)

 where
 having
 select
 group
 order
 limit
 offset
 joins
 includes (:include)
 lock
 readonly
 from

 Lazy loading

 Operations on relation
   published.new

   new(attributes)
   create(attributes)
   create!(attributes)
   find(id_or_array)
   destroy(id_or_array)
   destroy_all
   delete(id_or_array)
   delete_all
   update(ids, updates)
   update_all(updates)
   exists?

 scopes
   scope :published, where(:published => true)
   Reuse of scopes
   scope :published_since, lambda { |ago| published.where('created_at >= ?', ago) }

   Post.published_since(2.days.ago).all

 SQL
   Post.published_since(2.days.ago).to_sql
   puts  (Post.published & Post.where(:created_at => Date.today)).to_sql

 Reuse scopes across classes
   class Post < ActiveRecord::Base

     scope :published, lambda {
       where("posts.published_at IS NOT NULL AND posts.published_at <= ?", Time.zone.now)
     }
   end

 class User < ActiveRecord::Base

   scope :published, lambda {
     joins(:posts).
     where("posts.published_at IS NOT NULL AND posts.published_at <= ?", Time.zone.now).
     group("users.id")
   }
 end

 class User < ActiveRecord::Base

   scope :published, lambda {
     joins(:posts).group("users.id") & Post.published
   }
 end